1. Field of the Invention
The present invention relates to the field of electrical cable driver buffers. Specifically, the present invention relates to a circuit for effectuating a universal cable driver buffer that maintains a constant output voltage for a variety of cable termination electrical characteristics.
2. Related Art
Electronic communication media have become a ubiquitous and crucially important aspect of modern technology, commerce, industry, and leisure. Much modern electronic communications includes transmission of digital data. Some electronic communications media include cable-based modalities. One such cable-based communications modality effectuates electronic communications via coaxial cable. One coaxial cable standard for many communications applications has an impedance of 75 Ohms (xcexa9).
Communications via coaxial cable routinely utilize devices to place electronic signals, including digital data, onto and to pick such signals off from a coaxial cable. One such group of such devices are transmitter-receivers, also known as transceivers. In the design of components, such as a clock recovery integrated circuit (IC), needed for such transceivers, the intent is to empower the transceiver as a unitary device to drive, e.g., to pass signals effectively onto, a 75xcexa9 coaxial cable.
Drivers for coaxial cables employed in many communication applications are designed to comply with communications standards to assure interconnectivity among and between a plethora of communications networks and systems worldwide. One such standard with widespread adoption is ITU-G703, which is incorporated herein by reference. This standard is promulgated by the International Telecommunications Union (ITU) of Geneva, Switzerland. Standard ITU-G703 demands that compliant cable drivers impress a signal to be transmitted via a 75xcexa9 coaxial cable with an amplitude of one Volt (1 V).
P-channel 13.5 mA constant current sources are a conventional cable driver mainstay. These devices employ operational amplifiers (Op Amps) to achieve the signal gain required to effectively drive a 1 V signal on a 75xcexa9 coaxial cable. Referring to Conventional Art FIG. 1, one such op amp based cable driver is depicted. Such conventional cable drivers employ a bipolar implementation.
Their design approach incorporates complementary symmetry amplifiers. Such conventional drivers achieve an output voltage swing of twice the signal voltage specification. This is because compliance with telecommunications standards demands that, in order to properly terminate the communications cable, a termination resistor ROT, equal in ohmic resistance value to a source impedance ZOS, is connected in series with the line; the output voltage is accordingly divided between the two equal impedances, each dropping one half of the amplifier""s output signal voltage. For the steady state conditions of the present discussion, an impedance ZOC of the coaxial cable itself is negligible.
Such conventional cable drivers are switchable, such that their output current may be delivered to either of two loads. As depicted in Conventional Art FIG. 1, the output current of a cable driver may be delivered from either an inverting or a non-inverting source output terminal to respective load resistors ROT or ROT by parallel coaxial cable runs. Users of these drivers and the connected coaxial cables delivering the currents being driven by the drivers have the option of compliantly utilizing one or both of two differential signals with no penalty. In one option, a user elects to utilize one single-ended signal. In this case, to remain compliant with ITU-G703, the other available application must be terminated by an equivalent load resistance. In the present example, ROT and ROT"" are compliantly equal in ohmic resistance values.
The preferable method of compliantly terminating the cables delivering the output of a conventional driver is to utilize two separate 75xcexa9 resistors. In this case, the output signal may be taken from either resistor, switched between them, or taken from both and auctioneered, according to the preference of the user for a particular application. In any case, the 13.5 mA signal dropped across the 75xcexa9 resistor develops the 1 V output signal, in compliance with ITU-G703.
However, in using a constant current source to generate a signal with a 1 V amplitude, a problem arises with respect to parasitic capacitance. Parasitic capacitances arise at the output of the driver from a number of sources. Sources of parasitic capacitance there include (1) the capacitance of printed circuit boards used in the construction of both the drivers and the load; (2) routing capacitances arising from the layout of conductors carrying the signal, within the driver, the cable capacitance of coaxial cable itself, and within the load; (3) the capacitance arising between the bonding pads to which the load resistors are soldered or otherwise electrically coupled and mechanically mounted and the dielectric material constituting the material from which the load printed circuit boards are constructed; and (4) capacitance associated with all conductive copper, aluminum and/or metallic traces.
A charge-discharge loop is associated with the parasitic capacitances. The parasitic capacitance charges and discharges cyclically in accordance with the output signal. The charge-discharge cycle of the parasitic capacitance is deleterious for a number of reasons. One detrimental effect is that, with signals on the order of the amplitude under discussion herein, a non-negligible amount of current intended to be passed in the load is diverted to supplying the charging current.
Another effect of parasitic capacitance is adverse to the signal risetime and correspondingly degrades bandwidth and data transfer rate and capacity. Absent parasitic capacitance, the risetime is wholly dependent upon the signal itself, and follows from the device generating the signal. Parasitic capacitance however, in association with the load impedance, develops a charge-discharge time constant proportional to the capacitance, which adds delay to the signal risetime. In as much as bandwidth is inversely proportional to the signal risetime, the delayed signal risetime reduces bandwidth and the rate and capacity of data signal transmission accordingly.
The degradation of signal risetime due to parasitic capacitance is illustrated by reference to Conventional Art FIG. 2. In a first exemplary circuit with an associated parasitic capacitance of five pico Faradays (pF) has a corresponding signal risetime of just over 1 nanosecond (nS). A second exemplary circuit passes the identical 1 V amplitude signal as the first circuit. However, the second circuit has an associated parasitic capacitance of ten pF. It is seen that the signal risetime associated with the second circuit is twice that of the first circuit, between 2 and 3 nS.
In an effort to counter these detrimental effects, efforts must be taken to minimize parasitic capacitance. For instance, extreme care must be taken in the design layout of printed circuit boards used in drivers and loads and quality control of both materials selected for them and their construction. Also, extreme care is needed in the placement of load resistors and the routing and connection of cable. Further, connection of the cables, usually by BNC type connectors, adds to parasitic capacitance, each BNC connector adding a degree of capacitance and exacerbating the problem.
However, efforts at minimizing parasitic capacitance may be cumbersome and pose an undue burden on users and makers of cable drivers. Efforts such as exercise of care in printed circuit board layout and quality control are burdensome and expensive. Further, they may not effectuate all applications and in others may not suffice.
A further problem arises, which may be characterized as sensitivity to variations in load resistance inherent in conventional constant current drivers. With a constant 13.5 mA output current producing a 1 V peak-to-peak signal amplitude in compliance with ITU-G703, the load impedance is limited by Ohm""s Law to 75xcexa9. Any variation in load impedance, such as temperature-related resistance divergences will produce signal errors or departure from compliant tolerances.
Further, circuit designs become constrained by the load impedance strictures, preventing implementation of circuits with anything but precision 75xcexa9 load resistors. In some applications, this may deliver other than an optimal circuit design. Other applications may be precluded by the inflexibility of the load impedance stricture. Still others may be quite functional, yet suffer degraded performance due to factors causing variations in load impedance.
What is needed is a cable driver circuit and/or a method of driving a cable that is relatively insensitive to parasitic capacitance. What is also needed is a cable driver circuit and/or a method of driving a cable that is relatively insensitive to variations in load impedance. Further, what is needed is a cable driver circuit and/or a method of driving a cable that is able to source as much current as is demanded by a load having a relatively wide range of impedance values and any parasitic capacitances associated with the cable, the routing of the signal, and the connections at both cable ends, while maintaining a 1 V peak-to-peak signal amplitude, in compliance with ITU-G703.
The present invention provides a novel method for a driving a cable and a cable driver circuit that are relatively insensitive to parasitic capacitance. The present invention provides a cable driver circuit and a method of driving a cable that is relatively insensitive to variations in load impedance. Further, the present invention provides a cable driver circuit and a method of driving a cable that is able to source as much current as is demanded by a load, which may have a relatively wide range of impedance values and parasitic capacitances associated with the cable, the routing of the signal, and the connections at both cable ends, while maintaining a 1 V peak-to-peak signal amplitude, in compliance with the ITU-G703 specification.
In one embodiment, a source-follower circuit with a complementary metal oxide semiconductor (CMOS) implementation effectuates a cable driver circuit, which needs no operational amplifier for its functionality. In one embodiment, the cable driver circuit utilizes an internal precision voltage reference with a two-stage CMOS differential voltage amplifier, and a CMOS current mirror to generate a constant current source. The resulting constant current source delivers a signal compliant with the ITU-G703 specification, and which is stable and compliant over a wide range of load impedance values and associated capacitive milieus.